Astronomers say they've found what may be the smallest and most distant planet known to be orbiting a star outside our own solar system.
The work suggests that such small rocky or icy planets may be more common in the cosmos than Jupiter-sized gas giant planets, researchers said. The discovery also indicates the power of a relatively new method of finding such "exoplanets."
All of the exoplanets discovered so far around distant stars are larger than Earth. The newly found planet is about 5.5 times the mass of Earth, making it much smaller than most of the 160 previous exoplanet discoveries.
It appears to be less massive than another small planet found recently, but because of uncertainties in estimating mass that isn't known for certain, the scientists said.
The finding was reported in the journal Nature by a team of researchers from 12 countries.
Similarly, at a distance of more than 20,000 light years from Earth, it is probably the most distant such planet yet found, said study co-author David Bennett of the University of Notre Dame. But distance estimates are too uncertain to be highly confident of that, he said.
In any case, the planet appears to be much too cold to sustain life, probably reaching no more than minus 364 degrees, the researchers said. It orbits its star about about 2.6 times the distance between Earth and the sun.
The planet lies in the constellation Sagittarius, close to the center of the Milky Way.
Nearly all the known exoplanets have been detected by their gravitational tug on the stars they orbit, which makes the stars wobble.
The new planet is the third to be uncovered by a different technique, which uses the fact that a celestial body's gravity bends light like a cosmic glass lens.
If a planet and its star pass between Earth and a more distant star, this "microlensing" effect gives a temporary but telltale boost to the brightness of the more distant star.
This technique opens the door to finding relatively small planets with masses and orbits similar to Earth's, researchers said.
It has uncovered only two huge Jupiter-size planets so far, and it probably would have found dozens by now if they were as common as smaller planets, Bennett said. So it appears the small planets are much more common than the big ones, he said.
Jean-Philippe Beaulieu of the Astrophysical Institute of Paris, another author of the paper, said he expected that in the future, the international team that found the new planet "will get a handful like this per year."
The astronomers named the planet OGLE-2005-BLG-390Lb. "OGLE" is the group of astronomers who monitor stars for evidence of microlensing.